Soybean contains a substance, or a class of substances, which can markedly reduce the rate of growth of non-contact inhibited cells (L and HeLa cells ) in culture. Three tenths of a microgram of the partly purified substance reduces the growth rate of L cells in suspension culture by more than 85% without evidence of cytotoxic effects. The active component is soluble in chloroform-methanol (2:1); for this and other reasons it is believed that the active component is a complex glycolipid that exerts its action at the cell surface. Studies which will elicit the chemical composition and the mode of action of the active substance should help in our understanding of molecular events relating the cell membrane to cell division. Also, it may assist in the development of cancer chemotherapeutic agents or regimens.